The present invention relates to optical encoders and, more particularly, to encoders which may be used to provide print control information in a dot matrix copier such as an ink jet copier for copying all types of materials, including printed texts and photographs.
In order to utilize an ink jet printer as the printing mechanism for a copier, it is necessary to have some means of scanning the print density of the master image to produce print control information indicative of the pattern in which ink drops are deposited on the print medium. It is known in the prior art to illuminate the master image and then to direct light reflected from one or more rows of scan points on the image to a plurality of photoelectric transducers. The output signals from each transducer provides an indication of the print density of the master image at the associated scan point on a scan row. A large amount of light being reflected from a point on the master image will indicate a lighter toned area having little or no ink deposits.
An arrangement is provided for sweeping the rows of scan points across the master image. Electrical print control signals are thereby produced by the transducers which define the entire master image with sufficient resolution that a copy of the image may be printed by an ink jet printer.
The optical scanning arrangement for an encoder is usually determined, to a degree, by the jet printer configuration. In U.S. Pat. No. 3,604,846, issued Sept. 14, 1971, to Behane et al, an encoder is shown in which a single jet deposits drops upon a sheet of copy paper mounted on a rotating drum. The jet control circuit receives print information from an optical encoder in which a transparent master is scanned in a raster fashion. Relatively complicated data handling circuitry is required in the Behane et al device to convert the raster scan format into the proper data arrangement for use by the printer.
Another scanning arrangement is shown in U.S. Pat. No. 3,928,718, issued Dec. 23, 1975, to Sagae et al. In the Sagae et al device, the master is placed on a slowly rotating drum. The optical scanner is slowly moved axially along the rapidly rotating drum, thus scanning a plurality of points in parallel lines on the master image. A single jet printer is used in Sagae et al. A sheet of copy paper is mounted on a drum rotating at the same speed as the drum upon which the master image is mounted. The printer is moved axially along this drum in synchronism with the movement of the scanner. It is apparent that such an arrangement simplifies data timing and eliminates reordering of the data since the scanning and printing formats are identical and scanning and printing are accomplished simultaneously.
In order to provide images of sufficient resolution, it is necessary to deposit a large number of drops of ink on the print medium in precise registration. It will be appreciated that the speed at which printing is accomplished can be greatly increased by using a large number of jets positioned along a row which accomplishes printing in a single pass of the row of jets across the print medium. The minimum interjet spacing in an ink jet recorder is such that it may be difficult to position the jets as close together in a single row as would be desired for a given resolution, especially where the jets are operating in a binary fashion with each jet serving only a single row of print positions across the print medium. As seen in U.S. Pat. No. 3,913,719, issued Oct. 21, 1975, to Frey, one technique to overcome the limitation imposed by a minimum interjet spacing is position the jets in two parallel rows. The jets in the rows are staggered so that the ink drops deposited by the jets in the first row interlace with those deposited by jets in the second row. It will be appreciated that a data delay must necessarily be supplied in the data circuit providing print information to the second row in order to insure proper registration between the drops deposited by the two rows.
U.S. Pat. No. 3,560,641, issued Feb. 2, 1971, to Taylor et al discloses a printer in which plural tandem arrays of ink jet nozzles receive print information from a scanner having similarly positioned arrays of scanning elements. The master image is placed on a rotating drum and the arrays of scanning elements are positioned at predetermined locations around the drum periphery. While providing proper timing for the nozzle arrays, the Taylor et al scanner is somewhat cumbersome. Additionally, printers have been developed in which plural inclined rows of jets are used. This scanner does not lend itself readily to a configuration in which the rows of jet nozzles are positioned along lines which are inclined to the direction of print medium movement or in which a single row services plural inclined rows of print positions.
Accordingly, it is seen that there is a need for a simple, accurate scanning mechanism which provides print information for tandem rows of print positions serviced by an ink jet printer and which may be used with printers having drops which are deposited at print positions along lines which are inclined to the direction of print medium movement.